Complexes of bovine serum albumin with sulphated polysaccharides: effects of pH, ionic strength and high pressure treatment

Solutions of bovine serum albumin (BSA) (5 mg ml−1) subjected to high pressure processing (600 MPa for 20 min) at low ionic strength and neutral pH have shown a reduction in protein surface hydrophobicity. This decreases further in the presence of ι- or κ-carrageenan (2.5:1 weight ratio). The total calorimetric enthalpy (ΔH) for the pressure treated BSA is reduced by 50%. Addition of ι- or κ-carrageenan to BSA reduces both the denaturation temperature (Tm) and ΔH under the same treatment conditions. Size exclusion chromatography at low ionic strength has indicated a weak electrostatic interaction for BSA+ι- or κ-carrageenan at pH 7, which becomes stronger at pH 6.5. Complexation of BSA with the more highly sulphated ι-carrageenan gives a stronger electrostatic interaction than with κ-carrageenan under the same solution conditions. Replacement of ι-carrageenan with dextran sulphate (DS) gives an even stronger complex, which again suggests that the strength of complexation is dependent on the charge density on the polysaccharide. Complexation of BSA with polysaccharide at low ionic strength appears to protect the globular protein against pressure-induced aggregation. The addition of salt dissociates the protein–polysaccharide complex(es), and the protective effect of polysaccharide is then lost.